CN108732387A - A kind of probe sample distance controlling method and system of SICM - Google Patents

Abstract

The present invention relates to the probe sample distance controlling method and system of SICM a kind of.Method includes：Failing edge penalty function model is established before scanning；Upper pixel column scanning height is recorded in imaging process, the predicted value as current row to be scanned；It is input to failing edge penalty function model using the difference of probe real time position and pre-computed altitude as parameter, obtains failing edge penalty coefficient；Rising edge failing edge judgement is carried out, update obtains new current deviation；New current deviation is fed back into controller, control piezoelectric ceramics moves up and down, make probe kept always with sample it is permanent away from.System includes：Approximating curve module, failing edge penalty function model building module, row prediction module, feedback control module.The present invention solves the non-linear caused sample rise and fall of approximating curve along deviation different problems, thus can eliminate the failing edge blooming of SICM imagings, and it is more accurate to be imaged compared with using traditional PID control.

Description

A SICM probe sample distance control method and system

technical field

The invention relates to the technical field of scanning ion conductance microscope (SICM) imaging, in particular to an adaptive control method and system for the distance between a probe and a sample during SICM imaging.

Background technique

Scanning ion conductance microscope (SICM) is a scanning probe microscope widely used in biology, medicine, chemistry, materials and other fields. Compared with traditional optical microscope, it has higher resolution. The microscope is limited by the optical diffraction limit, and the resolution cannot reach below 200nm, while the scanning probe microscope uses the interaction between its probe and the sample surface as the imaging basis, and is not affected by the optical diffraction limit, and the resolution can reach several nanometers or tens of nanometers. Nano; Compared with other scanning probe microscopes, SICM is characterized by the non-destructive observation of living cells under physiological conditions without force contact, and does not require pretreatment of samples such as fluorescent labeling, so it is a high-resolution, High-fidelity imaging technology is especially suitable for the observation of soft samples such as living cells. SICM uses an ultra-micro hollow glass tube with a tapered tip and an inner radius of tens to hundreds of nanometers as a probe. The sample to be tested is placed in a sample dish filled with electrolyte solution, and the probe is also filled with The same solution, and an AgCl electrode is placed in the sample vessel and the probe, and a voltage is applied to both ends of the electrode to form a current loop together with the electrolyte solution to generate an ionic current. When the distance between the needle tip and the sample surface is far away, the ion current remains at a constant value. When the probe is close enough to the sample, the narrow gap between the needle tip and the sample will hinder the flow of ions and reduce the ion current. There is a one-to-one curve relationship between the distance between the probe and the sample, so the current can be used to characterize the distance between the probe and the sample surface. When imaging the sample, the probe is scanned across the surface of the sample line by line, the magnitude of the current is monitored in real time, and the movement of the probe in the vertical direction is controlled by negative feedback to maintain the ion current at a fixed value, which is equivalent to the probe in the The sample surface maintains a constant height, so the three-dimensional shape of the sample surface can be obtained by recording the trajectory of the probe movement.

The existing SICM probe sample distance control method is ordinary PID control. In a large number of imaging experiments using SICM on samples, it was found that the image formed by SICM has a tailing phenomenon, which is manifested as when a certain PID parameter is used. The rising edge of the height shows the real shape of the sample, and the image shape is longer than the actual shape at the falling edge of the height, that is, the probe tracks the height change in time on the rising edge, and the image shape on the falling edge Probes are slow to track changes in sample height. This phenomenon affects the real reflection of the sample morphology by SICM, which makes the scanning image deviate from the real morphology to a certain extent. Through analysis, it is found that the tailing phenomenon is caused by the nonlinearity of the current approximation curve. The approximation curve is the curve of ion current changes during the process of the probe gradually approaching the sample. When scanning, the maximum value in the approximation curve is set at 95% to 99%. fixed value to keep the current constant at the set value. Due to the non-linearity of the approximation curve, for the same height change, the current deviation as the feedback value is relatively large at the rising edge of the sample height, and the current deviation is small at the falling edge. If the same PID parameters are used at this time, the Different control values are generated, so that the speed of the probe tracking is different. The change of the sample height can be quickly tracked on the rising edge, but it takes longer to complete the tracking of the height on the falling edge, resulting in a tailing phenomenon.

Contents of the invention

In order to solve the problem of trailing edge trailing in SICM imaging caused by the nonlinearity of the approximation curve, the present invention provides an adaptive control method and system for the probe/sample distance.

The technical scheme adopted in the present invention is as follows: a method for self-adaptive control of the distance between a probe and a sample of SICM, comprising the following steps:

Step 1. Establish an approximation curve before scanning. The approximation curve is the relationship curve between the distance between the probe and the sample and the ion current; use the approximation curve to establish a falling edge compensation function model. The model is based on the Z axis between the current position of the probe and the position of the working point The piezoelectric ceramic voltage difference is the input, and the falling edge compensation coefficient is the output;

Step 2. During the scanning process, the scanning height of the previous line of the image is used as the working point position of the current line to be scanned;

Step 3. When scanning the current line, collect the real-time position of the piezoelectric ceramic, and substitute the Z-axis piezoelectric ceramic voltage difference between the current position of the probe and the predicted working point position into the falling edge compensation function to obtain the falling edge compensation coefficient;

Step 4. Determine whether the current position is a height falling edge; if so, multiply the ratio of the ion current deviation by the falling edge compensation coefficient and feed it back to the controller;

Step 5, the controller outputs the control amount to the piezoelectric ceramics, so that the probe tracks the shape height of the sample and keeps a constant distance from the surface of the sample.

The establishment of the falling edge compensation function model by using the approximation curve is specifically: using the approximation curve to establish the falling edge compensation function by fitting, and the output of the function is the multiple that the current deviation of the falling edge needs to increase.

The establishment of the falling edge compensation function model using the approximation curve includes the following steps:

Step 1-2-1. Taking the current setting value during image scanning as the middle point, divide the approximation curve into two parts: left and right, that is, height rise and height fall;

Step 1-2-2. In the two parts of height rise and height fall, the absolute value of the current deviation between the ion current and the current setting value is used as the function value, and the Z-axis piezoelectric ceramic voltage and current setting value are used. The absolute value of the corresponding Z-axis piezoelectric ceramic voltage difference is an independent variable, which is used to represent the distance from the current position of the probe to the working point. The change curve of the value; the current deviation of the rising edge is a positive deviation, and the current deviation of the falling edge is a negative deviation;

Step 1-2-3, using the ratio of the absolute value of the positive deviation to the negative deviation as the function value, taking the Z-axis piezoelectric ceramic voltage difference reflecting the distance from the current position of the probe to the working point as the independent variable, and using the least square method Coefficients are obtained by rational number fitting, and a falling edge compensation function is established.

The falling edge compensation function models the

k(d)=(p1×d+p2)/(d+q1)

Among them, k(d) is the falling edge compensation coefficient, d is the Z-axis piezoelectric ceramic voltage difference reflecting the distance between the current position of the probe and the working point, and p1, p2, q1 are coefficients.

Described step 4 comprises the following steps:

Step 4-1, collecting ion current and piezoelectric ceramic voltage in real time;

Step 4-2, judging whether the current scanning point is the rising edge or the falling edge of the sample height;

Step 4-3. If it is a rising edge, the current deviation keeps the original value; otherwise, the current deviation is updated to the original current deviation, multiplied by the falling edge compensation coefficient k, and then fed back to the controller.

Said step 4-2 comprises:

Step 4-2-1, respectively setting an ion current deviation value and a piezoelectric ceramic voltage difference value as the dead zone;

Step 4-2-2. When the sum of the current deviations exceeds the dead zone, it is judged as the falling edge of the sample height, otherwise it is judged as the rising edge of the height.

An adaptive control system for the distance between a SICM probe and a sample, comprising:

Approach curve module: set the probe approach stop point, and obtain the approach curve data;

Falling edge compensation function model building module: set the scanning working point, use the working point as the midpoint to draw the curve of the current deviation with the distance from the probe to the working point through the approximation curve, and get the positive and negative deviation ratio and reflect the probe The relationship curve of the Z-axis piezoelectric ceramic voltage difference between the current position and the working point position, and the establishment of the falling edge compensation function model;

Row prediction module: save the height information of each point in the current row, and provide it to the control module as the predicted value of height when scanning the next row;

Feedback control module: Use the falling edge compensation function model to process the current deviation to obtain a new deviation, input the new deviation to the PID controller to obtain the control value, and output the control value to the piezoelectric ceramic to control the height of the probe to track the shape of the sample surface .

The present invention has the following advantages:

1. The present invention can eliminate the falling edge blur phenomenon in SICM imaging. Due to the nonlinearity of the approximation curve, the rising edge and falling edge of the sample height produce different current deviations under the same distance deviation, which leads to different control quantities, making the speed of the probe tracking the shape slower on the falling edge, resulting in Blurring of the falling edge. The present invention increases the current deviation at the falling edge, making it the same size as the current deviation at the rising edge corresponding to the same distance, and solves the current deviation difference between the rising and falling edges of the sample height, so that the falling edge can obtain the same probe tracking Speed, when a certain PID parameter is set to make the image of the rising edge clear, the falling edge is also clear.

2. The present invention can avoid oscillation caused by excessive PID parameters. Using the original control method and system, when it is found that there is general ambiguity at the falling edge of the sample during the scanning process, it will be estimated that the imaging ambiguity is caused by the slow probe tracking speed, which will increase the PID parameters to speed up the response speed of the probe to the deviation, and at the same time, a greater amount of control will be generated at the rising edge, which increases the possibility of overshoot and oscillation, which is not conducive to obtaining the real shape, and even damages the sample. After using the present invention, the rising edge and the falling edge correspond to the same distance and deviation output with the same control amount, which avoids the occurrence of the above situation, makes it easier to obtain the real shape, and has a positive protective effect on the sample and the instrument at the same time.

3. The present invention enables the imaging system to distinguish pit features with a smaller width. For the pit shape with small width, due to the slow tracking speed on the falling edge, the probe has swept through its width area before tracking the depth of the pit, which makes it appear in the image as a deeper depth than the actual depth. Shallow pits even appear flat. The invention speeds up the response speed of the probe to the falling edge, so it has more accurate imaging capability for this type of shape.

Description of drawings

Fig. 1 is the flowchart of the distance self-adaptive control method between SICM probe and sample of the present invention;

Fig. 2 is a closed-loop system control block diagram of the present invention;

Fig. 3 a is the curve diagram of the current amplitude change during the process of the probe approaching the sample of the present invention, that is, the approximation curve diagram;

Fig. 3b is a curve diagram of the current deviation amplitude during the process of the probe approaching the sample of the present invention;

Fig. 3c is a curve diagram of the falling edge compensation function model of the present invention;

Fig. 4 is the flowchart of step 3 in the specific embodiment of the present invention;

Fig. 5 is the contrast diagram of the response characteristic curve of the method of the present invention and common PID control method to height step;

Figure 6a is the imaging result using a common PID control system;

Figure 6b is the imaging result using the adaptive distance control system;

Figure 6c is a cross-sectional graph of two imaging results.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

A method for adaptively controlling the distance between a SICM probe and a sample, comprising:

Step 1. Make an approximation curve before starting scanning. The approximation curve is the relationship curve between the probe/sample distance and the ion current. Use the approximation curve to establish a falling edge compensation function model. The model uses the Z of the current position of the probe and the position of the working point The voltage difference of the axial piezoelectric ceramic is the input, and the output is the ratio of the current deviation between the rising edge and the falling edge corresponding to the same distance;

Step 2. During the scanning process, record the scanning height of the previous line of the image as the predicted height of the current line to be scanned;

Step 3. When scanning the current row, collect the real-time position of the piezoelectric ceramic, and substitute the Z-axis piezoelectric ceramic voltage difference between the current position of the probe and the predicted working point position as a parameter into the falling edge compensation function model to obtain the falling edge compensation coefficient ;

Step 4. Determine whether the current position is a falling edge of altitude, and if so, multiply the feedback current deviation by the falling edge compensation coefficient as a new deviation in the feedback loop and send it to the controller;

Step 5. The controller outputs the control amount to the actuator piezoelectric ceramics, so that the probe tracks the shape height of the sample and maintains a constant distance from the sample surface.

Said step 1 includes:

Step 1-1. Obtain the overall approximation curve of the relationship between the probe/sample distance and the current, which reflects the entire process of ion current reduction from the maximum value to close to zero;

Step 1-2, using the approximation curve to establish a falling edge compensation function model by means of fitting, and the output of this function is the multiple that the current deviation of the falling edge needs to increase.

The steps 1-2 include:

Step 1-2-1, taking the current setting value (95%-99%) during image scanning as the middle point to divide the approximation curve into two parts, the left and right, that is, the height rise and the height fall;

Step 1-2-2. In the two parts of height rise and height fall, the absolute value of the current deviation between the ion current and the current setting value is used as the function value, and the Z-axis piezoelectric ceramic voltage and current setting value are used. The absolute value of the corresponding Z-axis piezoelectric ceramic voltage difference is an independent variable, and the variation curves of the current deviation of the rising edge and the falling edge with the Z-axis piezoelectric ceramic voltage difference are drawn respectively;

Step 1-2-3, using the ratio of the absolute value of the positive deviation to the negative deviation as the function value, taking the Z-axis piezoelectric ceramic voltage difference between the current position of the probe and the working point as the independent variable, and using the least square method to simulate Combined to get the coefficients, and establish the falling edge compensation function model.

Said step 3 includes:

Step 3-1, collecting the voltage of the piezoelectric ceramic in real time to represent the current position of the probe;

Step 3-2, subtracting the current position of the probe from the piezoelectric ceramic voltage at the predicted height position of the point to obtain a voltage difference representing the distance between the current position and the predicted working point position;

Step 3-3. Substitute the voltage difference between the current position of the probe and the predicted working point as an independent variable into the falling edge compensation function model to obtain the falling edge compensation coefficient K.

Said step 4 includes:

Step 4-1, collecting ion current and piezoelectric ceramic voltage in real time;

Step 4-2, judging whether the current scanning point is the rising edge or the falling edge of the sample height;

Step 4-3: If it is a rising edge, the current deviation remains unchanged; if it is a falling edge, update the current deviation to be the original current deviation multiplied by the falling edge compensation coefficient K.

Said step 4-2 comprises:

Step 4-2-1, setting an ion current deviation value and a piezoelectric ceramic voltage difference as the current dead zone and the voltage dead zone;

Step 4-2-2. When the current deviation and the voltage difference exceed the current dead zone and the voltage dead zone respectively, it is judged as the falling edge of the sample height, otherwise it is judged as the rising edge of the height.

A SICM self-adaptive control system for the distance between a probe and a sample, comprising:

Approach curve module: set the probe approach stop point, and obtain the approach curve data;

Falling edge compensation function model building module: set the scanning working point, use the working point as the midpoint to draw the curve of the current deviation with the distance from the probe to the working point through the approximation curve, and get the positive and negative deviation ratio and reflect the probe The relationship curve of the Z-axis piezoelectric ceramic voltage difference between the current position and the working point position, and the establishment of the falling edge compensation function model;

Row prediction module: save the height information of each point in the current row, and provide it to the control module as the predicted value of height when scanning the next row;

Feedback control module: Use the falling edge compensation function model to process the current deviation to obtain a new deviation, input the new deviation to the PID controller to obtain the control value, and output the control value to the piezoelectric ceramic to control the height of the probe to track the shape of the sample surface .

The present invention is a new control method for the distance between the SICM probe and the sample, as shown in Figure 1: the falling edge compensation function model is established through the approximation curve, combined with the line prediction method, the falling edge compensation coefficient is updated in real time, and then the falling edge position The current deviation is increased to the same level as the rising edge, eliminating the effect of the nonlinearity of the approximation curve on the imaging of the falling edge. Figure 2 is a block diagram of the control loop of the system.

The self-adaptive control method of the distance between described a kind of SICM probe sample comprises the following steps:

Step 1. Make an approximation curve before starting scanning. The approximation curve is the relationship curve between the probe/sample distance and the ion current. Use the approximation curve to establish a falling edge compensation function model. The model uses the pressure between the current position of the probe and the working point position The voltage difference of the electroceramic is input, and the ratio of the corresponding rising edge and falling edge current deviation is the output;

Said step 1 includes:

Step 1-1, as shown in Figure 3a, obtain the overall approximation curve of the relationship between the probe/sample distance and the current, set the stop condition of the probe approximation as the ion current reaches 10%-30% of the maximum current, and the curve reflects the ion The whole process of the current decreasing from the maximum value to close to zero;

Step 1-2, using the approximation curve to establish a falling edge compensation function model by means of fitting, and the output of the model is the multiple that the current deviation of the falling edge needs to increase.

The steps 1-2 include:

Step 1-2-1. Use the current set value to be used during image scanning as the middle point, which is the cut-off point. The set value is usually 95% of the current when the distance between the probe and the sample surface is relatively long (the current is constant and maximum). -99%, divide the approximation curve into left and right parts, the left side represents the situation of the falling edge of the sample height, and the right side represents the situation of the rising edge of the sample height;

Step 1-2-2, as shown in Figure 3b, take the absolute value of the current deviation between the ion current and the current setting value as the function value, and use the Z-axis corresponding to the Z-axis piezoelectric ceramic voltage and current setting value The absolute value of the piezoelectric ceramic voltage difference is an independent variable, which is used to represent the distance from the current position of the probe to the working point, and the variation curves of the current deviation of the rising edge and the falling edge with the Z-axis piezoelectric ceramic voltage difference are drawn respectively. , represents the relationship between the current deviation and the piezoelectric ceramic voltage difference when the probe is on the rising and falling edges of the sample;

Step 1-2-3, as shown in Figure 3c, use the ratio of the absolute value of the positive deviation to the negative deviation as the function value, and use the Z-axis piezoelectric ceramic voltage difference reflecting the distance from the current position of the probe to the working point as the independent variable , use the least squares method to fit rational numbers to get the coefficients, and establish the falling edge compensation function:

k(d)=(p1×d+p2)/(d+q1) (1)

Among them, k(d) is the falling edge compensation coefficient, d is the Z-axis piezoelectric ceramic voltage difference reflecting the distance between the current position of the probe and the working point, and p1, p2, q1 are coefficients, which are obtained by fitting rational numbers through the approximation curve.

Step 2. During the scanning process, record the piezoelectric ceramic voltage representing the scan height of a pixel row on the image as a prediction of the row height of the pixel to be scanned currently.

h _p (x,y)=h(x,y-1) (2)

Where (x, y) is represented as the pixel point of row x and column y in the image, h _p is the predicted height, and h is the scanning height;

Step 3. When scanning the current line, collect the real-time position of the piezoelectric ceramic, and substitute the voltage difference between the predicted target position and the current real-time position of the piezoelectric ceramic as a parameter into the falling edge compensation function model to obtain the falling edge compensation coefficient, as shown in Figure 4 . If the current scanning behavior is the first line of the image, only the ordinary PID control method is used for scanning, and the process of line prediction is not performed;

Said step 3 includes:

Step 3-1, collecting the voltage of the piezoelectric ceramic in real time represents the current position h _{z_piezo} of the probe;

Step 3-2. Subtract the height of the current position from the predicted height of the point to obtain the distance d between the current position and the position of the working point, expressed by the voltage difference of the Z-axis piezoelectric ceramic:

d(t)=h _{z_piezo} (t)-h _p (x,y) (3)

Step 3-3. Substituting the distance between the current position and the working point as an independent variable into the falling edge compensation function model to obtain the falling edge compensation coefficient k.

Step 4. Determine whether the current position is a falling edge of altitude, and if so, multiply the feedback current deviation by the falling edge compensation coefficient as a new deviation in the feedback loop and send it to the controller;

Said step 4 includes:

Step 4-1, collecting ion current and piezoelectric ceramic voltage in real time;

Step 4-2, judging whether the current scanning point is the rising edge or the falling edge of the sample height;

Said step 4-2 comprises:

Step 4-2-1. In order to avoid the influence of noise and the influence of height mutation between two rows, a current dead zone i _deadzone is set, and a piezoelectric ceramic distance dead zone d _deadzone is set to avoid the influence of piezoelectric ceramic vibration;

Step 4-2-2, Falling edge judgment condition:

Step 4-3. If it is a rising edge, the current deviation keeps the original value unchanged; if it is a falling edge, update the current deviation to be the original current deviation multiplied by the falling edge compensation coefficient k.

e(t)=k(d(t))×(i(t)-i _sp ) (5)

Among them, upward and downward respectively represent the rising edge and falling edge, e(t) represents the new current deviation, i(t) represents the feedback amount, that is, the ion current, and i _sp represents the current setting value.

Step 5. The controller outputs the control amount to the actuator piezoelectric ceramics, so that the probe tracks the shape height of the sample and maintains a constant distance from the sample surface.

In order to verify the control performance of the present invention on the distance between probes and samples when using SICM system imaging, the response characteristic experiment to the height step and the imaging experiment on the standard PDMS grid sample were carried out. During the scanning process, the sample is manually given a downward height step, and the above-mentioned adaptive control method and ordinary PID control method are used respectively to record the process of the probe tracking the height change and record the response characteristic curve. Use the nanometer platform to make the sample produce a 1 micron falling edge, as shown in Figure 5, use ordinary PID and adaptive control to let the probe track it, the PID parameters used in the two cases are the same, the ordinary PID control method probe tracking The trajectory is a curve with a small slope on the right, and the time to complete the tracking is greater than 100ms. The trajectory tracked by the probe using the adaptive control method is a curve with a large slope on the left, and the time to complete the tracking is less than 10ms. Therefore, the distance between the samples of the SICM probe is used The adaptive control method makes the probe track the falling edge faster, which is obviously better than the ordinary PID control method.

Fig. 6a-Fig. 6c is the scanning result of the grid under the control system of the present invention and the scanning result comparison diagram under the common PID control system; Fig. 6a-Fig. The PID parameters used are the same. The grid sample used for scanning is 5 μm wide and 200 nm deep, and the material is polydimethylsiloxane (PDMS). The image scanned using ordinary PID control is clear at the rising edge of the left grid, but blurred at the falling edge of the right, as shown in Figure 6a; the imaging effect of the image scanned using adaptive control at the rising edge of the left grid is the same as that The PID control is consistent, but it is clearer at the falling edge on the right, eliminating the tailing phenomenon, as shown in Figure 6b. Cutting the two images at the same position, it shows that the method of the present invention makes the height curve at the falling edge of the grid steeper than the original method, and is closer to the real shape, as shown in Figure 6c, which verifies its control performance advantage.

Claims (7)

1. a distance self-adaptive control method between the probe of SICM and the sample, it is characterized in that comprising the following steps: Step 1. Establish an approximation curve before scanning. The approximation curve is the relationship curve between the distance between the probe and the sample and the ion current; use the approximation curve to establish a falling edge compensation function model. The model is based on the Z axis between the current position of the probe and the position of the working point The piezoelectric ceramic voltage difference is the input, and the falling edge compensation coefficient is the output; Step 2. During the scanning process, the scanning height of the previous line of the image is used as the working point position of the current line to be scanned; Step 3. When scanning the current line, collect the real-time position of the piezoelectric ceramic, and substitute the Z-axis piezoelectric ceramic voltage difference between the current position of the probe and the predicted working point position into the falling edge compensation function to obtain the falling edge compensation coefficient; Step 4. Determine whether the current position is a height falling edge; if so, multiply the ratio of the ion current deviation by the falling edge compensation coefficient and feed it back to the controller; Step 5, the controller outputs the control amount to the piezoelectric ceramics, so that the probe tracks the shape height of the sample and keeps a constant distance from the surface of the sample. 2. the distance self-adaptive control method between the probe of a kind of SICM and the sample by claim 1, it is characterized in that, described utilizing approximation curve to set up falling edge compensation function model is specifically: utilize approximation curve by the mode of fitting A falling edge compensation function is established, and the output of the function is the multiple that the current deviation of the falling edge needs to increase. 3. by a kind of probe of SICM described in claim 1 and 2 and distance self-adaptive control method between sample, it is characterized in that, described utilizing approximation curve to set up falling edge compensation function model comprises the following steps: Step 1-2-1. Taking the current setting value during image scanning as the middle point, divide the approximation curve into two parts: left and right, that is, height rise and height fall; Step 1-2-2. In the two parts of height rise and height fall, the absolute value of the current deviation between the ion current and the current setting value is used as the function value, and the Z-axis piezoelectric ceramic voltage and current setting value are used. The absolute value of the corresponding Z-axis piezoelectric ceramic voltage difference is an independent variable, which is used to represent the distance from the current position of the probe to the working point. The change curve of the value; the current deviation of the rising edge is a positive deviation, and the current deviation of the falling edge is a negative deviation; Step 1-2-3, using the ratio of the absolute value of the positive deviation to the negative deviation as the function value, taking the Z-axis piezoelectric ceramic voltage difference reflecting the distance from the current position of the probe to the working point as the independent variable, and using the least square method Coefficients are obtained by rational number fitting, and a falling edge compensation function is established. 4. a kind of SICM probe according to claim 1 and distance self-adaptive control method between sample, it is characterized in that, described falling edge compensation function model k(d)=(p1×d+p2)/(d+q1) Among them, k(d) is the falling edge compensation coefficient, d is the Z-axis piezoelectric ceramic voltage difference reflecting the distance between the current position of the probe and the working point, and p1, p2, q1 are coefficients. 5. by the distance self-adaptive control method between the probe of a kind of SICM and sample according to claim 1, it is characterized in that, described step 4 comprises the following steps: Step 4-1, collecting ion current and piezoelectric ceramic voltage in real time; Step 4-2, judging whether the current scanning point is the rising edge or the falling edge of the sample height; Step 4-3. If it is a rising edge, the current deviation keeps the original value; otherwise, the current deviation is updated to the original current deviation, multiplied by the falling edge compensation coefficient k, and then fed back to the controller. 6. a kind of SICM probe according to claim 5 and distance self-adaptive control method between sample, it is characterized in that, described step 4-2 comprises: Step 4-2-1, respectively setting an ion current deviation value and a piezoelectric ceramic voltage difference value as the dead zone; Step 4-2-2. When the sum of the current deviations exceeds the dead zone, it is judged as the falling edge of the sample height, otherwise it is judged as the rising edge of the height. 7. An adaptive control system for distance between a SICM probe and a sample, characterized in that it comprises: Approach curve module: set the probe approach stop point, and obtain the approach curve data; Falling edge compensation function model building module: set the scanning working point, use the working point as the midpoint to draw the curve of the current deviation with the distance from the probe to the working point through the approximation curve, and get the positive and negative deviation ratio and reflect the probe The relationship curve of the Z-axis piezoelectric ceramic voltage difference between the current position and the working point position, and the establishment of the falling edge compensation function model; Row prediction module: save the height information of each point in the current row, and provide it to the control module as the predicted value of height when scanning the next row; Feedback control module: Use the falling edge compensation function model to process the current deviation to obtain a new deviation, input the new deviation to the PID controller to obtain the control value, and output the control value to the piezoelectric ceramic to control the height of the probe to track the shape of the sample surface .